1. Field of the Invention
The present invention relates to a camera having an automatic focusing function and particularly to an auto focus camera having a zooming mechanism.
2. Description of the Prior Art
In a conventional zoom lens, a distance between a variator lens and a compensator lens is generally regulated by a cam ring so that a focusing position of light from an object may be fixed independent of zooming operation. In such a zoom lens, focusing operation is performed by using a front lens.
The U.S. Pat. No. 2,782,683 discloses that a position of a variator lens and that of a compensator lens are detected by using sliding resistance, whereby a positional relation between the variator lens and the compensator lens is controlled without using a cam ring. In this U.S. Patent, sliding resistance means for detecting the positions of the lenses is required in place of the cam ring.
In addition, the Japanese Patent Publication No. 15226/1977 discloses a zoom lens which compensates for a shift of focus by controlling a position of a focus lens according to absolute focal length information of the zoom lens when zooming is effected after temporary manual focusing. However, this zoom lens is not associated with an auto focus apparatus. Accordingly, zooming operation is to be performed after the focus has been temporarily adjusted manually. In addition, absolute focal length information of the zoom lens is required and the position of the focus lens, not the moving speed thereof, is controlled. Therefore, this zoom lens cannot always ensure an in-focus condition during zooming.
As described above, in a zoom lens, a distance between a variator lens and a compensator lens is generally controlled by a cam ring so that a focusing position of light from an object may be fixed independent of zooming operation. However, in order to make a lens have a small size and a light weight, it is desirable to ensure an in-focus condition during zooming without using such a cam ring.
In a zoom lens for a single-lens reflex camera, an in-focus condition needs to be set only at the time of releasing the shutter and it is not necessary to maintain the in-focus condition during zooming. Accordingly, it is only necessary to control the position of the compensator lens based on the result of focus detection. However, in a video camera, zooming is effected during photographing operation and it is necessary to constantly maintain an in-focus condition during zooming.
On the other hand, there have been proposed several focus correction systems in which zooming is performed by a variator lens and movement of a focus position caused by the zooming is corrected by movement of a compensator lens. Types of such correction systems differ in what the movement of the compensator depends on. For example, the Japanese Patent Laying-Open No. 92127/1975 indicates that the compensator is moved based on an output of a focus detecting apparatus. On the other hand, the Japanese Patent Laying-Open No. 143309/1985 discloses that position information of the compensator necessary for setting an in-focus condition dependent on a change in a focal length caused by a movement of the variator is stored in a ROM and the position information is read out corresponding to the position of the variator so as to be used for control of the movement of the compensator.
However, the above described method of the Japanses Patent Laying-Open No. 143309/1985 requires a ROM of a large capacity for attaining a high precision of focus correction and this method is not a practical method.
On the other hand, the method described in the Japanese Patent Laying-Open No. 92127/1975 is simple in construction since it is only necessary to detect a defocus condition of a focal plane and to drive the compensator based on the detected condition. However, if this method is applied to a video camera, the following disadvantages are involved. More specifically, if focus correction (compensation for a shift of focus) during zooming is considered, dynamic correction characteristics (that is, those characteristics during zooming) becomes particularly important to the video camera as described above. This will be explained with reference to FIG. 1. A TTL passive type focus detecting apparatus as disclosed in the Japanese Patent Laying-Open No. 4914/1985 for example, that is, a focus detecting apparatus of the so-called phase difference detection system is well known as a focus detecting apparatus for detecting a defocus condition on a focusing surface. However, this detecting apparatus is of a charge integration or accumulation type and it causes a time lag corresponding to a period elapsed after integration a charge representing an amount of light from an object until an end of focus detecting calculation. Such time lag is also caused in a focus detecting apparatus of a contrast type. FIG. 1 represents a follow-up or tracking characteristic of a compensator during zooming in a focus compensated apparatus using a focus detecting system causing such time lag, in which zooming is effected from an end of a telephoto range (referred to hereinafter as the tele end) in a direction opposite to the tele end (referred to hereinafter as the wide direction, an end opposite to the tele end of the telephoto range being afterwards referred to as the wide end and a direction toward the tele end being afterwards referred to as the tele direction). The abscissa in FIG. 1 represents a elapse of time after the zooming operation, not a focal length and the ordinate represents a movement amount of the compensator. In the figure, the curve A is an ideal follow-up curve of the compensator and the lines B indicate a follow-up locus of compensator based on a signal of the focus detecting apparatus. It is assumed that the curve A shows a characteristic exhibited when the distance D to the object is D=.infin.. The waveform P shown below the abscissa represents and integration time control pulse for focus detection. A period of a high level of the pulse corresponds to an integration time and a period of a low level thereof corresponds to a focus detecting calculation time. Although the integration time is actually changed dependent on luminance of an object, it is assumed in the figure that the integration time is fixed for simplification of the explanation.
Referring in FIG. 1, assuming that the compensator is first located at a position corresponding to the time T0, the variator is in a stopped state at the time T0 and the compensator is in a stopped state with an in-focus condition being maintained by the focus detecting apparatus. Then, when zooming is started in the wide direction, the variator is moved in the wide direction and focus detection is continued until the time T1. Assuming that zooming and integration are started simultaneously for the purpose of simply explaining the focusing operation based on detection of a defocus amount during zooming, only the variator is moved in the wide direction with the compensator being stopped in a time period from T0 to T1. Accordingly, at T1 when the integration and the detecting calculation are terminated, the focus detecting apparatus detects an amount corresponding to DF1 in FIG. 1 as a defocus amount at an integration baricentric position during the integration (i.e., a center of the high-level period of the pulse P). Therefore, there is a time lag corresponding to a period from defocusing to detection of the defocus amount. When the defocus amount DF1 is detected at the time T1, the focus compensated (AF) apparatus enables the compensator to start to be moved at a given speed by an amount corresponding to DF1. It is assumed in this case that the given speed is equal to a focus control speed at the time of normal AF operation without zooming and corresponds to the maximum inclination of the ideal follow-up curve A. It is further assumed that after the time T1, focus detection is effected by moving the compensator so as to maintain a good follow-up characteristic.
As described above, the movement of the compensator is started when the defocusing is first detected at the time T1. If a defocus amount larger than DF1 is already caused at this time, the compensator is moved by the detected defocus amount DF1 and after that, an in-focus condition is regarded as being set and the movement of the compensator is stopped until the time T2. After the time T1, the subsequent integration is started and a defocus amount DF2 is detected at the time T2 in the same manner, whereby the compensator starts to be moved by an amount corresponding to DF in the same direction at the time T2. Then, at the time T3, a defocus amount DF3 is detected and integration is effected while the compensator is being moved. When correction for an amount already obtained, that is, evaluation of DF3-DF3' is effected, the compensator is stopped again at a position shown in the figure at the time T4. In the same manner, defocus amounts DF4 to DF9 are detected at the times T4 to T9, respectively, and the compensator is moved and stopped repeatedly in the same direction at the same speed as previously set. As a result, a large error is caused by delays in follow-up as shown in the figure and a motion picture obtained is of a considerably poor quality. This drawback becomes particularly conspicuous in the case of a video camera. However, even in the case of a conventional camera with film, the same phenomenon occurs if pictures are successively taken during zooming. In order to minimize the error in follow-up, it is possible to shorten each detection cycle of the focus detecting apparatus. However, for that purpose, it is necessary to increase the sensitivity of a light receiving element to reduce an integration time, or to use a processing unit having a high speed of calculation. In such cases, the costs are unavoidably increased. Furthermore, if such measures are taken, it is impossible in the above described conventional focus detection method to effectively reduce the errors in follow-up as shown in FIG. 1. Particularly, it is difficult to avoid an excessive defocus caused by a delay in follow-up at the start of zooming or a delay in follow-up at the time of reversing the zooming direction. A method for improving the follow-up characteristic is proposed for example in the Japanese Patent Laying-Open No. 264307/1986, in which moving speeds of a compensator are preset for the respective zones of distances to an object and the compensator is controlled by selecting any of the preset moving speeds at the time of zooming. Although this method is effectively applied when zooming is made in the same direction, it does not take account of other cases such as reversing of the zooming direction, a rapid change in information of distance to the object or a case in which focus detection is incapable. Thus, in such cases, the follow-up characteristic is deteriorated and it takes time to attain suitable focus correction.
In addition, cameras of the latest models perform focus detection in AF operation and a TTL passive type phase difference detection system, a contrast system and the like are adopted in those cameras. According to such systems, charge generated by incidence of light from an object is stored to perform integration and an output of the integration is used for focus detection. Such systems have an advantage that a sufficiently large detection signal suitable for processing can be obtained. However, the output value of the integration attains a predetermined level rapidly or at a considerably slow speed, dependent on luminance of the object. If the output value of the integration attains the predetermined level at a too slow speed, the follow-up characteristic with respect to the object is deteriorated and a shutter chance is often missed. In view of those disadvantages, the Japanese Patent Laying-Open No. 260261/1986 discloses that if an output of integration does not attain a predetermined level within a predetermined period, the integration operation is stopped, so that gain correction is made with an amplification degree according to the output of the integration.
However, this prior art takes no account of a focus operation relationship with a zooming operation. During zooming, there are brought about various conditions such as a change in an object image, blurring of picture due to an unsteady hold on the camera, or a difficulty in focusing on an object. If the integration time in a focus detection means is long, the tendency of deviation or the difficulty in focusing on the object is further increased. As a result, errors are produced in the integration output with high probability or the number of focus detection cycles repeated per unit time is decreased to cause deterioration in the follow-up characteristic for compensation for a change in a focusing position. Thus, particularly in the case of a video camera, an out-of-focus condition is often brought about during zooming and an unsatisfactory video image is obtained. In addition, even in a conventional camera with film, unsatisfactory results are brought about when pictures are successively taken during zooming.
In addition, there is proposed a camera for manually selecting a focus detection area for AF operation and also manually selecting a focus detection area indicating pattern to be displayed in a finder. However, such a manual selection system involves troublesome operation and it further has a disadvantage in that it is difficult to set a suitable focus detection area according to a situation of an object. Under the circumstances, there is proposed a camera for automatically selecting a focus detection area and a focus detection area indicating pattern to be displayed in a finder according to a situation of an object, for example, when focus detection is incapable.
However, such a camera for automatically selecting a focus detection area according to a situation of an object involves disadvantages as described below during zooming operation for moving a variator to change a focal length in a zoom lens unit. During zooming, compared with a stopped state of the variator, a view angle frequently changes to make it difficult to catch an object within an focus detection area located for example in the center of a photographing area, and blurring of picture due to an unsteady hold on the camera is liable to occur. Further, light from an object applied to a light receiving element for focus detection becomes unstable due to the change of the view angle, causing irregularities in the results of focus detection and it often happens that focus detection is incapable. If focus detection is incapable, switching is made to focus detection area having a different range and a focus detection area indicating mark is changed corresponding thereto. If such switching occurs frequently, it presents considerable difficulties in observing images in the finder.
In addition, if such switching occurs frequently, a focus detection time is prolonged by a period required for the switching, causing a further lowering in focusing precision.